1. Field of the Invention
The invention proceeds from a current-limiting component having an electrical resistance body arranged between two contact terminals and containing first resistance material, which material has PTC behavior and a low cold resistivity below a first temperature and forms at least one current-carrying path extending between the two contact terminals and which material has a high hot resistivity compared with its cold resistivity above the first temperature.
2. Discussion of Background
Resistors having PTC behavior have already been prior art for a long time and are disclosed, for example, in DE 2 948 350 C2 or U.S. Pat. No. 4,534,889 A. Such resistors always contain a resistance body composed of a ceramic or polymeric material which has PTC behavior and conducts electrical current well below a limit temperature specific to the material. PTC material is, for example, a ceramic based on doped barium titanate or an electrically conductive polymer, for instance a thermoplastic, semicrystalline polymer, such as polyethylene, containing, for example, soot as conductive filler. When the limit temperature is exceeded, the resistivity of the resistor based on a PTC material increases abruptly by many orders of magnitude.
PTC resistors can therefore be used as overload protection for circuits. Because of their limited conductivity (carbon-filled polymers have, for example, a resistivity of more than 1 .OMEGA..multidot.cm), they are generally limited in their practical application to rated currents of up to approximately 8 A at 30 V and up to approximately 0.2 A at 250 V.
J. Mat. Sci. 26 (1991), 145 ff. provides PTC resistors based on a polymer filled with borides, silicides or carbides which have very high conductivity at room temperature and which could in principle be used as current-limiting components even in power circuits involving currents of, for example, 50 to 100 A at 250 V. Resistors of this type are, however, not commercially available and cannot therefore be produced without appreciable effort.
If a PTC resistor is used as current-limiting protective component in an electrical network designed for high operating currents and high operating voltages, appreciable energy is converted in the PTC resistor during the turn-off process if a short circuit occurs. In particular, if the turn-off process takes place nonuniformly in the PTC resistor, this can result in the PTC resistor forming locally overheated regions, so-called "hot spots", approximately in the center between the contact terminals. In the overheated regions, the PTC resistor switches to the high-resistance state earlier than at the unheated points. The entire voltage applied across the PTC resistor then drops over a relatively small distance at the point of the highest resistance. The high electrical field associated therewith may then result in breakdown and in damage to the PTC resistor.